Field of the Invention
The invention relates to a three-point converter and to a method of operating the three-point converter.
The invention is associated with the field of three-point converters that are fed by voltage intermediate circuits. Converters of this type are used both in electrical drives and in filter and compensation installations involving high power levels. The protective circuit proposed is, in particular, a possibility for the realization of high-power converters in the medium-voltage range.
A topology of a neutral point clamped (NPC) three-point converter has already been generally known for a long time. It is also used industrially in the field of high power levels. IGBTs, GTO thyristors or integrated gate commutated thyristors (IGCT) are used as main switches in this case.
When GTO thyristors are used, it is necessary to limit the rate of current rise di/dt during turn-on and also the rate of voltage rise du/dt during turn-off across the switches.
When IGCTs are used, it is necessary to limit only the rate of current rise di/dt. For this purpose, switching load-relief networks, so-called snubbers, are disposed in the circuit; they ensure the abovementioned limitation and thereby generally reduce switching losses in the switches.
A number of examples of such configurations are described in Suh, J.-H. et al.: "A New Snubber Circuit for High Efficiency and Overvoltage Limitation in Three Level GTO Inverters", IEEE Trans., On Industrial Electronics, Vol. 44, No. 2, April 1997. The limiting of the rates of voltage and current rise is achieved, in principle, by capacitors connected in parallel with the switches and, respectively, inductors connected in series as well as additional active and/or passive components which are always necessary. When GTO thyristors are used, the protective circuits must generally be implemented separately for each GTO or for each phase. The outlay on active and passive components is therefore high. Both in the case of conventional RCD snubbers and in the case of the improved variant proposed, the entire energy stored in the snubber is converted into heat via resistors. Problems which arise when conventional RCD snubbers are used, such as overvoltages across the GTO thyristors (caused by the series inductors) and unequal blocking voltage distribution between the GTOs, can only be minimized, but not solved, even with the improved snubber mentioned above.
An example of an industrially manufactured snubber is described in Komulainen, R.: "Inverter Protected in Respect of the Rates of Increase of Current and Voltage", U.S. Pat. No. 4,566,051, published on Jan. 21, 1986. In this circuit, the entire energy (snubber energy) stored in the load-relief inductors and load-relief capacitors is fed back into the DC voltage intermediate circuit. However, this requires a high outlay on circuitry.
A further disadvantage of all known switching load-relief networks for three-point converters is that the switching losses due to the high commutation voltage (half the intermediate circuit voltage in the three-point converter) can be reduced only to a limited extent and with a high outlay on components.
For the topology of two-point converters, in addition to the diverse snubber circuits which are likewise used, principles are also known in which low-loss switching is achieved by a commutation voltage of almost zero. For this purpose, an electrical network for temporarily decoupling the commutation voltage from the intermediate circuit voltage during the switching operations is incorporated between the intermediate circuit capacitor and the bridge paths of the converter, as described e.g. in Salama, S, Tadros, Y.: "Quasi Resonant 3-Phase IGBT Inverter", IEEE-PESC Conference Records, 1995. In this case, the commutation voltage can oscillate almost to zero by a resonant operation.
To summarize, it may be ascertained that the snubber circuits used to date in conventional NPC three-point converters ensure only a limited reduction of switching losses in the main switches on account of the functional principle of the snubber circuits. In many snubbers, the snubber energy is not fed back into the DC voltage intermediate circuit but rather is converted into a heat loss in the switches and the protective circuit. Furthermore, the outlay on components and the costs of conventional snubbers are considerable.